Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. In recent investigations, Peripheral Stimulation (PS) (i.e., stimulation of nerve tissue outside of the spinal cord and brain), which includes Peripheral Nerve Field Stimulation (PNFS) techniques that stimulate nerve tissue (and in particular, nerve endings, defined to be the distal ends of the neurons which are made up of the receptors (mechanical, chemical, thermal, non-specific, etc) and the axonal regions that connect the receptors to the main axon traversing to the central nervous system, these axonal regions being typically several mm in length) directly at or near the symptomatic site of the disease or disorder (e.g., at the source of pain), and Peripheral Nerve Stimulation (PNS) techniques that directly stimulate bundles of peripheral nerves (in particular, neural axons) that may not necessarily be at the symptomatic site of the disease or disorder, has demonstrated efficacy in the treatment of chronic pain syndromes (e.g., painful peripheral neuropathy (PN), post-herpetic neuralgia (PHN), fibromyalgia syndrome (FMS), failed back surgery syndrome (FBSS), Arachnoiditis, occipital neuralgia, peripheral pelvic pain, cardiac pain, etc.) and incontinence, and a number of additional applications are currently under investigation.
An implantable neurostimulation system, whether used in the context of PS or another stimulation application, typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site. In PS, the stimulation lead(s) are implanted in the subcutaneous tissues of a peripheral region, such as the lower back region, cervical region, arm, or leg. With respect to PNS, the electrode or electrodes are placed directly on or in close proximity to a particular nerve, whereas with respect to PNFS, the electrode or electrodes are placed in a painful area without respect to a particular nerve's location). The implantable neurostimulation system further includes a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted within a tissue pocket remotely from the stimulation site, but coupled to the stimulation lead(s). Thus, electrical pulses can be delivered from the neurostimulator to the stimulation lead(s) to stimulate or activate a volume of neural tissue. In particular, electrical energy conveyed between at least one cathodic electrode and at least one anodic electrodes creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neurons beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers.
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen. The regimen will dictate which of the electrodes are sourcing current pulses (anodes) and which of the electrodes are sinking current pulses (cathodes) at any given time, as well as the magnitude, duration, and rate of the current pulses. The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. In the case of PS, such a therapeutic benefit is accompanied by “paresthesia,” i.e., a tingling sensation that is effected by the electrical stimuli applied through the electrodes.
With respect to PNFS, it is desirable that the entire target tissue (i.e., the nerve tissue responsible for the pain) be stimulated. However, it is generally not practical to cover the entire target tissue with stimulating electrodes. Thus, since the target tissue will typically extend beyond the locations of the electrode or electrodes, the amplitude of electrical stimulation energy conveyed by these electrode(s) must be increased in order to expand the electrical stimulation field to the outer reaches of the target tissue; that is, to provide far field stimulation at neural locations that are far from the activated electrode(s). This results in wider regions of paresthesia sensation, which may relate to broader regions of pain relief. Oftentimes, because the amplitude of the electrical stimulation energy must be increased to stimulate neurons in the far field, this may result in a painful “pinch” sensation in the tissue (e.g., by stimulating A-delta fibers) adjacent the stimulation electrode(s). Thus, the stimulation coverage may be limited by the painful sensations experienced by the patient at the stimulation electrode(s).
There, thus, remains a need for an improved technique to perform PNFS in order to treat the entirety of the pain region while avoiding or minimize the occurrence of pain in the tissue adjacent the stimulating electrodes that may result from the stimulation itself.